HYDROMETER.. An instrument for ascertaining the specific gravities of different liquids. The most common description of these instruments consists of a hollow ball, of either metal or glass, capable of floating in any known liquid, and having two stems, the lower one terminating in a weight, in order that the instrument may float with the stems upright, and the upper stem (which is of the same diameter throughout), being graduated, to show the den sity of the fluid by the depth to which it rinks ; as the heavier fluids will buoy up the instrument more than such as are lighter. In this way, however, it is clear that the stem must be of considerable thickness, in order that the instru ment may have an extensive range, in which case the smaller differences of den sity will not be perceptible. To obviate this imperfection various contrivances have been resorted to ; one of the most common of which is, to construct the instrument as above described, but with a very slender stem, which is divided into 100 equal parts ; and to provide a number of movable rings, all of equal weight, any one of which being slipped over the stem, when the installment floats in distilled water with zero on the stem at the surface of the water, will cause it to descend until the top of the scale be at the surface; and the density is estimated by the number of weights required to bring the lower part of the scale below the surface, minus the number of divisions of the scale which remain above the surface. But the method of Fahrenheit is both simpler and more accurate. The hydrometer of Fahrenheit consists of a hollow ball with a counterpoise below, and a very slender stem above, terminating in a small dish. The middle or half length (the stem,) is distinguished by a fine line across, and the instrument is always immersed up to this line by placing weights in the little dish above. Then as the part immersed is constantly of the same magni tude, and the whole weight of the hydrometer is known, this last weight added to the weights in the dish will be equal to the weight of fluid displaced; and if the gravity of water be represented by 1000, and the weight be divided into thousandth parts of the weight of the instrument when it sinks to the middle of the stem in distilled water, the number of weights required to sink the instrument to the mark on the stem when floating in any fluid, added to the weight of the instrument, or 1000, will represent the specific gravity of the The engraving on the following page represents an instrument for ascertain ing specific gravities, invented by Dr. Hare, Professor of Chemistry in the
University of Pennsylvania. This instrument, to which the inventor has given the name of Litrameter, owes its efficiency to the principle, that when columns of different liquids are elevated by the same pressure, their heights must be inversely as their gravities. Two glass tubes, of the size and bore usually employed in barometers, are made to communicate internally with each other, and with a gum elastic bag G, by means of a brass tube, and two sockets of the same metal, into which they are severally inserted. The brass tube termi nates in a cock, in which the neck of the bag is tied. Between the cock and the glass tubes there is a tubo at right angles to an opening into that which connects them. At the lower end of this tube, a small copper rod R enters through a collar of leather. The tubes are placed vertically in grooves, against an upright strip of wood, tenonned into a pedestal of the same material. Parallel to one of the grooves, in which the tubes are situated, a strip of brass is fastened, and graduated, so that each degree may be equal to of the whole height of the tubes. The brass plate is long enough to admit of about 140°. Close to this scale a vernier o is made to slide, so that the divisions of the scale are susceptible of sub-division into tenths, and the whole height of the tubes into about 2200 parte or degrees.